Issue 22

H. Singh et alii, Frattura ed Integrità Strutturale, 22 (2012) 69-84; DOI: 10.3221/IGF-ESIS.22.08 78 velocity. It is examined that the calculated velocity of a 12 µm copper particle can be increased from 553 m/s to 742 m/s, with a 33% increase in particle velocity by increasing the length of the nozzle from 83mm to 211mm, with nitrogen as the carrier gas. This increased velocity leads to an increase in the deposition efficiency from less than 10% to close to 80%. However, there are fabrication and material constraints that limit the practical length of the nozzles. So new materials need to be tried to improve powder flow through the nozzle and optimization in design is required to minimize the gas flow through the nozzle [20]. Karthikeyan [21] used specially designed tungsten carbide nozzle for coating on a special alloy GRCop-84 and Champagne et al. [22] successfully used thermoplastic nozzle to mitigate the effect of clogging of steel nozzle by Al particles. Fig. 11 shows the optimum value of 5 mm of nozzle exit diameter for maximum acceleration of particles with different diameters using nitrogen gas at a pressure of 2MPa and temperature of 300 o C. Li et al. [23], designed a short cold spray gun nozzle for applications in limited internal diameters and calculated the optimal design of expansion ratio of 6.25 with nozzle divergent section length of 40mm for nitrogen or helium gas at the standoff distance of 30mm and found that dense coating can be deposited by the designed short spray gun. Figure 11 : Effect of nozzle exit diameter on velocity of particles of different sizes using nitrogen at a pressure of 2MPa and temperture of 330°C [23] Effect on Microstructure The particle velocity (Vp) also affects the microstructure of the cold sprayed coating .The first layer of the particles on the surface is tamped or ram down hard by the high velocity particles for successive layer and the top layer remain porous as compared to inner region having dense microstructure. The thickness of this top porous layer is influenced by the spray conditions, material properties and the morphology of the particles. Since this particle velocity greatly increased with helium as accelerating gas as compared to nitrogen gas under the same operating conditions, so a better tamping effect is reported by Li et al. [13] with helium gas, resulted in a thinner top layer than using nitrogen. It is also reported the large top porous layer of titanium coating by cold spray using nitrogen as accelerating gas; even though the deposition efficiency is larger than 80%. D EPOSITION EFFICIENCY eposition efficiency (DE) is one of the most important characteristics of the cold spray coating process. It is the efficiency of deposition of spray powder on the substrate surface and it is practically impossible to obtain 100% deposition efficiency [7], due to the complicated nature of this spray process. Deposition efficiency can be calculated experimentally as kd = Δms/Mp where Δms is change of weight of a substrate and Mp is weight of all particles interacting with a substrate [7]. DE depends upon many factors: delay time, angle of impact of spray particles on the substrate surface, critical velocity, spray powder morphology and substrate surface characteristics like area of the contact surface, crater depth, plastic strain, yield stress, pressure and temperature at the contact boundary, etc [7]. One of the important factors which effect the deposition of particles on the substrate or DE, is the critical velocity Vc. As practically, only the particles having reached a velocity larger than the critical velocity can be deposited to produce coating. Therefore, the critical velocity and particle velocity prior to impact determine the deposition efficiency under a given spray D